Impact Of Bag Position On Temperature Uniformity
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Placing a bag (or in the experiment an ice-brick) horizontally near the water surface led to a temperature difference between bottom and top surface of the ice-brick of several degrees Celsius, due to evaporation cooling at the water surface. Experiments 'Layout' The water bath used was a FreshMealsMagic immersion heater with its original polycarbonate container with 15L water controlled by a SousVideMagic 1500D v3. An ice-brick of 560g/-20°C was attached either horizontally or vertically to a skewer fitting into the bath, the two K-type wire-probes of a Voltcraft K202 datalogging thermometer (resolution 0.1°C) were attached to both sides of the brick. To avoid floating, the brick was tied down with a wire threaded through the grid of the FMM heater. Temperatures were logged every 10 seconds. Before mounting the ice-brick, the temperature probes were positioned in the bath near the sensors of the SousVideMagic of a Greisinger GMH3710 high precision thermometer (resolution 0.01°) and calibrated against the display of the SousVideMagic. 'Results' 'Uncovered bath, position horizontal, no forced circulation' The bottom side of the ice-brick came up to set temperature within 12 minutes, the top side remained about 1.5°C below the temperature of the bottom side. Due to water evaporation which was about 0.15L/h, the water layer above the brick became shallower and the temperature difference between top and bottom side gradually increased to 3°C. 'Uncovered bath, position vertical, no forced circulation' Both sides of the ice-brick came up to set temperature within 41 minutes, whereafter the temperature difference between the two sides of the brick was about 0.2°C with some oscillation. 'Covered bath, position horizontal, no forced circulation' Both sides of the ice-brick came up to set temperature within 19 minutes, whereafter the temperature difference between the two sides of the brick was about 0.1-0.2°C with some oscillation. 'Uncovered bath, position horizontal, forced circulation (FMM-bubbler)' Both sides of the ice-brick came up to set temperature within 9 minutes, whereafter the temperature difference between the two sides of the brick was about 0.1-0.2°C with some oscillation. After 4 hours, water evaporation had shallowed the water layer above the brick from 1.5cm to 0.6cm, and the temperature at the bottom (!) side started oscillating. 'Uncovered bath, position vertical, forced circulation (FMM-bubbler)' Both sides of the ice-brick came up to set temperature within 10 minutes with a slight overshoot for about 20 minutes, whereafter the temperature difference between the two sides of the brick was about 0.1°C with some oscillation. Conclusions *An uncovered water bath may lead to uneven temperature distribution by evaporation and consequent evaporation cooling of the superficial water layers. *Uneven temperature distribution happens especially with insufficiently submerged items in horizontal position. Recommendations *An open water bath should be covered by a plastic cover, hollow plastic balls (ping-pong balls), a styrofoam cover, or whatever, to avoid evaporation and consequent evaporation cooling of the superficial water layers. *Vertical positioning of bags avoids the insufficient submersion that may occur with horizontally placed bags, and in case of multiple bags, it does not impede natural convection. *A tall water bath as opposed to a shallow bath has the advantage of allowing even large pieces to be positioned vertically. *Forced circulation (pump or aquarium bubbler) is desirable especially in shallow water baths (useful water depth less than 20cm, like laboratory water baths or SousVideSupreme). *Bags floating horizontally in an unstirred and uncovered water bath must be avoided, as this may lead to uneven heating of the food, which is especially important in pasteurizing food. If horizontal placement is inevitable, adequate submersion must be ensured. Acknowledgments I would like to thank the following people for their helpful suggestions: e_monster, Robert Jueneman, blackp, Douglas Baldwin, and Frank Hsu. PedroG 09:53, July 8, 2010 (UTC)